A COMPARISON OF UPPER SILURIAN BRACHIOPOD FAUNAS OF THE ALEXANDER TERRANE OF SE ALASKA WITH THOSE OF NW BALTICA

In recent years it has become evident that the Alexander terrane of SE Alaska is of non-North American origin. This terrane has been postulated as being derived from the Klamath Mountains of northern California, the Tasman geosyncline of Australia, as well as the Baltica or Siberian paleocontinents. The Silurian age Heceta Limestone of the Alexander terrane contains a diverse array of brachiopod communities ranging from a deep-water basin to algal barrier reef facies to inner platform lagoons. Our study of the Heceta brachiopod fauna indicates that a Siberian or Baltica origin is most likely. We here compare and contrast the Heceta Limestone brachiopod fauna known from NW Baltica (including Lithuania, Latvia, Estonia, and Gotland). The Upper Silurian brachiopod fauna of the Alexander terrane is overall closely comparable with that of NW Baltica, with the exception that Gracianella is not recorded from NW Baltica. Another difference is the lack of chonetids and rareness of dalmanellids in the Heceta Limestone, which are very abundant in coeval rocks of NW Baltica. The comparison of brachiopods from the Alexander terrane and the Urals are based mainly upon the pentamerids. Overall, the SE Alaskan Late Silurian has a brachiopod content reminiscent of parts of the Uralian-Cordilleran Region. In particular, abundant Gracianella, a genus well known from the Eastern Urals, Carnic Alps and also Nevada, plus the presence of certain pentameroid genera known elsewhere chiefly from the Eastern Urals. These taxa are not recognized in China or Australia, or in the Mongolo-Okhotsk Province. Likewise, these taxa are not known from Europe or Laurentia during the Late Silurian. It is presently difficult to state whether the SE Alaskan brachiopod fauna belongs to the Eastern Urals or to Baltica. However, the lack of chonetids and the rarity of dalmanellids in SE Alaska and the E. Urals allow us to conclude that these two areas were much closer to each other, than with NW Baltica.